DE69710512T2 - liquid-crystal display - Google Patents

Description

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display and an improvement in contrast of a projection type liquid crystal display using a reflection type liquid crystal display element.

A projection type liquid crystal display according to a conventional technique produces a display by projecting light onto a screen, so that a brightness sufficient to recognize the display under the light of a house's illumination is required. In order to improve the brightness of the display without increasing the power consumption, it is also necessary to increase the aperture ratio. In order to increase the aperture ratio, a liquid crystal display element in which the upper part of a TFT (thin film transistor) is covered with a reflection electrode is suitable and can also be used for a display. In the conventional projection type liquid crystal display, twisted nematic liquid crystal is used for the liquid crystal layer, two polarizing plates are arranged in front of and behind the liquid crystal layer, and the display is carried out.

However, since the aperture ratio is low, sufficient brightness for viewing the display under the light of room lighting is not achieved. Similarly, even if a polarization beam splitter is used instead of the polarizing plate, sufficient brightness cannot be achieved. According to Japanese Patent Laid-Open No. 64-7021, a reflection plate is provided between the liquid crystal layer and the lower substrate. A liquid crystal display having this structure is hereinafter referred to as a built-in reflection plate type liquid crystal display.

In a liquid crystal display with a built-in reflection plate according to the conventional technique, since the wavelength dispersion of a polarization state of light transmitted through the liquid crystal layer is large, the polarization state of the light passing through the liquid crystal layer must be controlled over all visible wavelengths in order to obtain a high-contrast display.

However, a method for controlling the wavelength dispersion is not described in Japanese Patent Laid-Open No. 64-7021. Since the liquid crystal display of the reflection plate-built-in type has a high aperture ratio for the above-mentioned reason, it potentially offers the possibility of achieving the brightness sufficient for recognizing the display under the light of the room lighting.

Accordingly, one task was to achieve sufficient brightness and at the same time a sufficient contrast ratio. The features of the generic terms of patent claims 1 and 3 are known in combination from EP 699938 A.

SUMMARY OF THE INVENTION

It is therefore an object of the claimed invention to provide a liquid crystal display having excellent visibility while simultaneously satisfying brightness and contrast ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings. In the drawings:

Fig. 1 is a schematic diagram showing a pixel structure of a liquid crystal display according to an embodiment of the invention;

Fig. 2 is a diagram showing components of the liquid crystal display of Fig. 1 relating to the polarization state of transmitted light;

Fig. 3 is a diagram showing an optical system optically equivalent to Fig. 2;

Fig. 4 is a diagram explaining the composition of retardations when using two transparent birefringent media;

Fig. 5 is a diagram for explaining the wavelength dispersion of a total value of delays in the case of Fig. 4 and a delay curve of a quarter wavelength by comparison;

Fig. 6 is a diagram showing the dependence of the retardation of a liquid crystal layer on an applied voltage in an embodiment of the invention;

Fig. 7 is a cross-sectional view taken along the line A-A' showing the pixel structure of the liquid crystal display according to Fig. 1;

Fig. 8 is a cross-sectional view taken along line B-B' showing the pixel structure of the liquid crystal display according to Fig. 1;

Fig. 9 is a diagram showing the wavelength dispersion of each of the retardations of the phase plates 1 and 2 and a liquid crystal layer according to an embodiment of the invention;

Fig. 10 is a diagram showing the wavelength dispersion of a total value of the retardations of the phase plates 1, 2 and the liquid crystal panel at the time of dark display shown in Fig. 9 and a retardation curve of a quarter wavelength;

Fig. 11 is a diagram showing the dependence of reflectivity on applied voltage in Embodiment 1;

Fig. 12 is a diagram showing the wavelength dispersion of the total value of the retardations of the phase plates 1, 2 and the liquid crystal layer at the time of dark display in the embodiment 2 and the retardation curve of a quarter wavelength;

Fig. 13 is a diagram showing the dependence of reflectivity on applied voltage in Embodiment 2;

Fig. 14 is a graph showing the wavelength dispersion of the total value of the retardations of the phase plates 1, 2 and the liquid crystal layer at the time of dark display in the embodiment 3 and a retardation curve of a quarter wavelength;

Fig. 15 is a diagram showing the dependence of reflectivity on applied voltage in Embodiment 3;

Fig. 16 is a graph showing the temperature dependence of the reflectivity in a bright display and the reflectivity in a dark display and the contrast ratio in Embodiment 4;

Fig. 17 is a diagram showing the wavelength dispersion of the total value of the retardations of the phase plates 1, 2 and the liquid crystal layer at the time of dark display and the retardation curve of a quarter wavelength in a comparative example;

Fig. 18 is a graph showing the dependence of reflectivity on applied voltage in the comparative example;

Fig. 19 is a characteristic diagram useful for understanding the invention, showing the retardation of the wavelength dispersion of each of three liquid crystal display elements in a projection type liquid display;

Fig. 20 is a characteristic diagram useful for understanding the invention, showing some examples of the wavelength dispersion of retardations reducing the reflectivity of the projection-type liquid crystal display for the entire visible wavelength range;

Fig. 21 is a characteristic diagram showing the wavelength dispersion of the retardation of each of the phase plates and a single liquid crystal layer;

Fig. 22 is a characteristic diagram showing an example of the wavelength dispersion of the retardation obtained by synthesizing retardations of a phase plate and the liquid crystal layer;

Fig. 23 is a characteristic diagram showing an example of the wavelength dispersion of the retardation obtained by synthesizing retardations of two phase plates and the liquid crystal layer 24;

Fig. 24 is a characteristic diagram showing a reflection spectrum of a black display of a conventional projection type liquid crystal display;

Fig. 25 is a characteristic diagram useful for understanding the invention, showing the reflection spectrum of a black display of the projection type liquid crystal display;

Fig. 26 is a diagram useful for understanding the invention, showing the structure of a light source, an optical system, and the liquid crystal display elements of the projection type liquid crystal display;

Fig. 27 is a cross-sectional view of a liquid crystal display element used for the projection type liquid crystal display, useful for understanding the invention;

Fig. 28 is a characteristic diagram showing the total values of the delays at the time of black display of a projection type liquid crystal display according to Example 1;

Fig. 29 is a characteristic diagram showing the dependence of the surface brightness of the projection type liquid crystal display according to Example 1 on the applied voltage;

Fig. 30 is a characteristic diagram showing the dependence of the color tone of the projection type liquid crystal display according to Example 1 on the applied voltage;

Fig. 31 is a characteristic diagram showing the dependence of the surface brightness of the projection type liquid crystal display according to Comparative Example 1 on the applied voltage;

Fig. 32 is a characteristic diagram showing the dependence of the color tone of the projection type liquid crystal display according to the Comparative Example 1 on the applied voltage; and

Fig. 33 is a characteristic diagram showing the total values of the delays at the time of dark display of the projection type liquid crystal display according to Comparative Example 1.

PREFERRED EMBODIMENTS OF THE INVENTION

Embodiments of the invention are described below with reference to the drawings.

Fig. 1 is a diagram showing the structure of a pixel of a liquid crystal display according to an embodiment of the invention. It shows an example of a cross section of a liquid crystal display element of the reflection plate-built-in type according to the invention. A polarizing plate is used as a polarizing means for converting light from a light source as natural light into polarized light.

On the lower substrate, there is a TFT connected to a pixel electrode, which also serves as a reflection plate on said lower substrate. The liquid crystal layer is made of nematic liquid crystal, the alignment directions of the liquid crystal layer near the upper substrate and near the lower substrate are identical, and the rotation angle is 0º. The phase plates 1 and 2 and a polarization plate are stacked on the lower substrate.

Fig. 2 is a diagram showing components related to a polarization state of transmission light of the liquid crystal display according to Fig. 1. That is, the components related to the polarization state of light were extracted from Fig. 1, simplified and drawn.

As shown in Fig. 2, the polarizing plate, the phase plates 1 and 2, the liquid crystal layer and the reflection plate relate to the polarization state of light. The retardation axes of the phase plates 1 and 2 are parallel to or perpendicularly intersect the alignment direction of the liquid crystal layer. The angle between the absorption axis (or transmission axis) of the polarizing plate and the retardation axis of the phase plate or the alignment direction of the liquid crystal layer is set to 45°. The retardations of the phase plates 1 and 2 and the liquid crystal layer at a wavelength λ are expressed by ΔndPH1(λ), ΔndPH2(λ) and ΔndLC(λ), respectively, as shown in Fig. 2.

According to Fig. 2, light passes through the liquid crystal display element in sequence via the polarizing plate, the phase plate 1, the phase plate 2 and the liquid crystal layer and is reflected by the reflection plate. Then the light passes through the liquid crystal layer, the phase plate 2, the phase plate 1 and the polarizing plate in reverse order and reaches the user. A change in the polarization state of the light in this step can be achieved by changing the polarization state in the optical system according to Fig. 3.

Fig. 3 is a diagram showing the optical system optically equivalent to that of Fig. 2. This means that the transmission axes of the polarizing plates 1 and 2 are parallel and the retardations of both the liquid crystal layer and the phase plate 1 and the phase plate 2 are twice as large as those of Fig. 2.

The conditions for improving the contrast of the liquid crystal display element of the reflection plate-built-in type, i.e., the conditions for reducing the reflectance at the time of dark display are investigated using the optical system shown in Fig. 3. The reflectance R(λ) of the light with the wavelength λ in the optical system shown in Fig. 3 is expressed by the following equation.

R(λ) = 0.5(1 + cos(4πΔnd"(λ)/λ)) ... (1),

where Δnd"(λ) is a total value of the retardations of the liquid crystal layer, the phase plate 1 and the phase plate 2, which will be described below.

To reduce the reflectivity of the dark display, it is sufficient if R(λ) is approached to zero over almost the entire range of visible wavelengths, ie from 400 nm to 700 nm. This means that in the range of visible wavelengths from 400 to 700 nm, it is sufficient that the total value of the retardations Δnd"(λ) obtained when applying VB by combining the retardations of the liquid crystal layer, the phase plate 1 and the phase plate 2 substantially corresponds to Δnd'(λ) in the following equation when a At the time of the dark display, the voltage applied to the liquid crystal layer is set to VB, ie it can be said that R(λ) approaches zero when the second term of equation 1 is approximately equal to (-1).

?nd'(?) = (0.25 + 0.5 n)? ... (2)

where n is an integer.

Equation 2 shows a wavelength dispersion characteristic (hereinafter also abbreviated to wavelength dispersion) in which the retardation increases in proportion to the wavelength when n is 0 or more. If n is less than 0, a wavelength dispersion is shown in which the retardation decreases in proportion to the wavelength. The retardations of all transparent birefringence media known to date (for example, the phase plate, the liquid crystal layer and the like) are positive and show a wavelength dispersion in which the retardation decreases as the wavelength increases.

Then, it is attempted to obtain a retardation corresponding to equation 2 by a combination of the liquid crystal layer and the phase plate. When the transparent birefringence media 1 and 2, whose retardations at the wavelength λ are Δnd1(λ) and Δnd2(λ), respectively (Δnd1(λ) > Δnd2(λ)), are stacked so that the retardation axes are parallel, a total value of the retardations Δnd"(λ) of the two transparent birefringence media is determined by the following equation.

Δnd"(λ) = Δnd1 (λ) + Δnd2 (λ) ... (3)

When they are stacked so that the lags intersect perpendicularly, Δnd"(λ) is determined by the following equation.

Δnd"(λ) = Δnd1 (λ) - Δnd2 (λ) ... (4)

If the wavelength dispersion of the retardation of the transparent birefringent medium 1 is different from that of the medium 2, the wavelength dispersion of the total value of the retardations Δnd"(λ) is different from those of the two transparent birefringent media 1 and 2.

In other words, the wavelength dispersion of the total value of the retardations can be controlled by an appropriate combination of the retardations of the transparent birefringence media 1 and 2, the gradients of the wavelength dispersion of the retardations and the layering methods (the layering methods for setting the retardation axes parallel or perpendicular to each other).

By using such a total value of the delays, the wavelength dispersion of the total value of the delays Δnd"(λ) can be adjusted close to Equation 2.

In particular, in order to realize the wavelength dispersion of the retardations according to equation 2 with respect to the gradients of the wavelength dispersions of the retardations of the transparent birefringence media 1 and 2, one gradient must be set extremely steep and the other gradient must be set extremely flat and/or the retardations of the transparent birefringence media 1 and 2 must be increased.

At the same time, when using only one phase plate, one of the transparent birefringence media serves as liquid crystal layer and the other as a phase plate. However, in order to maintain the display characteristics, the gradient of the wavelength dispersion of the retardation of the liquid crystal layer cannot be extremely steep or extremely flat. Likewise, the retardation of the liquid crystal layer cannot be increased. Therefore, when only one phase plate is used, the gradient of the wavelength dispersion of the retardation of the phase plate can be set either larger or smaller than that of the liquid crystal layer. Accordingly, it can be said that the wavelength dispersion of the total value of the retardations cannot be sufficiently approximated to Equation 2.

On the other hand, since two phase plates are used in the present invention, the gradient of the phase plate 1 is set to be larger (steeper) than the gradient of the liquid crystal layer and the gradient of the phase plate 2 is set to be smaller (flatter) than the gradient of the liquid crystal layer, thereby enabling the wavelength dispersion of the total value of the retardations of the projection optical system incorporating the liquid crystal layer and two phase plates to be extremely close to the retardation curve expressed by the ratio (0.25 + 0.5 n)λ according to Equation 2. In other words, as mentioned above, with respect to the gradients of the wavelength dispersion of the retardations of the phase plates 1 and 2 serving as transparent birefringence media 1 and 2, one gradient must be set to be extremely larger (steeper) than that of the liquid crystal layer and the other gradient must be extremely flatter (smaller) than that of the liquid crystal layer.

The above is described using Figs. 4 and 5.

Fig. 4 is a diagram explaining the composition of the retardation value when using the two transparent birefringence media. Fig. 5 is a diagram showing the wavelength dispersion of the total value of the retardations in the case of Fig. 4 and the retardation curve of a quarter wavelength in comparison. An example is shown in which the wavelength dispersion of the total value of the retardations between the liquid crystal layer and a phase plate as two transparent birefringence media according to the conventional technique is compared with the retardation curve of the quarter wavelength.

More specifically, polysulfone, in which the gradient of the wavelength dispersion of the retardation is the steepest, is used for the phase plate. A retardation value of the liquid crystal layer at a wavelength of 550 (nm) is set to 200 nm at the time of dark display, and the retardation of the phase plate at a wavelength of 550 (nm) is set to 62.5 nm. The phase plate is set so that its retardation axis perpendicularly crosses the alignment direction of the liquid crystal. In this case, the total values of the retardations are shown by the hatched portion in Fig. 4. This is shown again in Fig. 5.

For comparison purposes, Fig. 5 also shows the wavelength dispersion of the retardation when in equation 2 (n = 0), i.e. the retardation curve of the quarter wavelength where in equation 1 R(λ) is zero.

It is clear from Fig. 5 that the wavelength dispersion of the total delays is not sufficiently close to Equation 2 when only one phase plate is used.

The invention, on the other hand, is characterized in that two phase plates are used to make the wavelength dispersion of the total value of the retardations close to Equation 2. This means that instead of the liquid crystal layer in which the retardation and the gradient of the wavelength dispersion · the retardation cannot be freely changed, a phase plate is added and two phase plates are used in which these values can be changed, thereby enabling a wide range of adjustments. The wavelength dispersion of the total value of the retardations of the three elements is brought close to the quarter wavelength retardation curve. The above can be said to be equivalent to a case in which the number of parameters to be suitably adjusted is increased from one to two.

In other words, the liquid crystal display according to the present invention comprises the liquid crystal layer, a driving device, a polarizing plate, two phase plates, and upper and lower transparent substrates. Each of the two substrates has a display electrode and an alignment film. The display electrode on the lower substrate also serves as a reflection plate. The display electrode on the lower substrate is connected to an active element as a driving device. The two substrates are arranged to face each other with the liquid crystal layer interposed therebetween. The polarizing plate is arranged on the upper substrate. The two phase plates are arranged between the polarizing plate and the upper substrate. The direction of the absorption axis of the polarizing plate and the alignment direction of the liquid crystal layer near the upper substrate are not parallel. When the two phase plates are the phase plates 1 and 2, n is an integer, λ is the wavelength of the light, and a voltage VB is selectively applied to the liquid crystal layer using the driving device, the wavelength dispersion of the total value of the retardations of the phase plate 1, the phase plate 2 and the liquid crystal layer over the visible wavelength range of 400 to 700 nm is substantially approximated to the retardation curve expressed by the relationship (0.25 + 0.5 n)λ.

When the retardation of the liquid crystal is set to ΔndLC(λ), the two phase plates are phase plates 1 and 2, and the retardations of phase plates 1 and 2 are set to ΔndPH1(λ) and ΔndPH2(λ), the total value of the retardations Δnd"(λ) of the liquid crystal layer and phase plates 1 and 2 is expressed by the following equation.

Δnd"(λ) = ΔndLC(λ) ± ΔndPH1(λ) ± and ΔndPH2(λ) ... (5)

where the double sign (the sign ±) is not sequential .

In equation 5, the signs of the second and third terms are + when the retardation axes of the phase plates 1 and 2 are parallel to the alignment direction of the liquid crystal layer, and the sign becomes - when the retardation axes perpendicularly intersect the alignment direction of the liquid crystal layer. An example of the wavelength dispersion of each of the retardations when using the liquid crystal layer and the two phase plates (...).

Fig. 9 is a diagram showing the wavelength dispersion of each of the retardations of the phase plates 1 and 2 and the liquid crystal layer in the embodiment of the invention.

As phase plates 1 and 2, a phase plate made of polysulfone and a phase plate made of polyvinyl alcohol are used. Their retardations at a wavelength of 550 (nm) are set to 331 and 408 nm, respectively. Phase plate 1 is arranged so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal. Phase plate 2 is arranged so that the retardation axis is parallel to the alignment direction of the liquid crystal. The retardation of the liquid crystal layer is set to 60 nm at a wavelength of 550 (nm). The results are shown in Fig. 10.

Fig. 10 is a diagram showing the wavelength dispersion of the total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer at the time of dark display shown in Fig. 9 and the retardation curve of a quarter wavelength. As shown in Fig. 10, the wavelength dispersion (characteristic curve) of the total value of the retardations increases with wavelength in a similar manner to Equation 2 and coincides with the retardation curve of the quarter wavelength at a wavelength of 550 nm. The wavelength dispersion is an extremely close line that approximates the retardation curve of the quarter wavelength in the visible wavelength region (in the range of 400 to 700 nm).

There are only two ways of setting the retardation axis of the phase plate, that is, the retardation axis is parallel to the alignment direction of the liquid crystal layer or intersects it perpendicularly. Therefore, when using three or more phase plates, the retardation axes of at least two of them are parallel, and the two phase plates whose retardation axes are parallel can be replaced by one phase plate. Therefore, even if three or more phase plates are used, there is not much difference in the essential effects from the case where two phase plates are used. The total value of the delays will not be closer to Equation 2 than shown in Fig. 10. Taking the cost increase into account, the optimal number of phase plates is 2.

When a polarization beam splitter is used as the polarization device, equations 1 and 2 are replaced by equations 6 and 7, respectively.

R(λ) = 0.5(1 - cos(4πΔnd"(λ)/λ)) ... (6)

?nd'(?) = 0.5n? ... (7)

The reason for this is that the oscillation direction of a transmission polarization component from the polarization beam splitter is rotated by 90º when the moving direction of the light is changed. The above statements are similarly satisfied in this case too. The optimum number of phase plates for improving the contrast ratio is two.

If the retardation of the liquid crystal layer at the time of dark display is sufficiently small, the change in contrast ratio associated with a change in temperature is also small. For example, if the retardation of the liquid crystal layer is changed by 5% due to a change in temperature, the amount of change in retardation is 10 nm when the retardation of the liquid crystal layer is 200 nm. If the retardation of the liquid crystal layer is 40 nm, the amount of change in retardation is only 2 nm.

This means that the setting is made so that dark display is performed when the retardation of the liquid crystal layer is small, thereby suppressing the change in the retardation of the liquid crystal layer due to a change in temperature. Accordingly, deterioration of the contrast ratio due to a change in temperature can be suppressed. In other words, it can be said that reducing the retardation of the liquid crystal layer at a wavelength of 550 (nm) at the time of dark display is a necessary condition for preventing deterioration of the contrast ratio due to a change in temperature.

It has been found that the retardation of the phase plate 2 at a wavelength of 550 (nm) becomes larger than the retardation of the phase plate 1 at a wavelength of 550 (nm) when performing the dark display with a sufficiently and suitably low value of the retardation of the liquid crystal layer, for example, 40 (nm) as described below in Tables 1 and 2. In other words, by setting the retardation of the phase plate 2 at a wavelength of 550 (nm) to be larger than the retardation of the phase plate 1 at a wavelength of 550 (nm), it is found that the retardation of the liquid crystal layer at a wavelength of 550 (nm) has a low value suitable for solving the problems.

Fig. 6 is a graph showing the dependence of the retardation of the liquid crystal layer on the applied voltage in the embodiment of the invention. As shown in the graph, the retardation of the liquid crystal layer is changed by the applied voltage. The ordinate in Fig. 6 shows the retardation obtained by dividing of the retardations by a value obtained at an applied voltage of 0 V. The retardation of the liquid crystal layer decreases drastically from 1 V to 2 V. When the dark display is performed on the side of a voltage of more than 2 V, a display with a high contrast ratio can be obtained without being affected by temperature.

Specific contents and effects of the invention are described below with reference to embodiments.

EMBODIMENT 1

Fig. 1 is a plan view showing an example of the structure of a pixel of a liquid crystal display according to Embodiment 1 and its periphery. Fig. 7 is a cross-sectional view showing the structure of a pixel of the liquid crystal display according to Fig. 1 along the line A-A'. Fig. 8 is a cross-sectional view showing the structure of a pixel of the liquid crystal display according to Fig. 1 along the line B-B'. The structure of the liquid crystal display according to Embodiment 1 will be described.

A lower substrate 10 is made of a non-alkaline borosilicate glass, with silicon oxide layers arranged on and under the substrate. A gate electrode 20 is made of an aluminum film produced by sputtering and on which an anodic aluminum oxide film is formed. An insulating film 25 is a silicon nitride film with a film thickness of 2000 Å produced by a plasma CVD method. A TFT operates in such a way that a channel resistance is increased by applying a positive bias voltage to the gate electrode is reduced and the channel resistance is increased by a zero bias voltage. The TFT is formed by the gate electrode 20, the insulating film 25 and an i-type semiconductor layer 30.

The i-type semiconductor layer is made of amorphous silicon and has a film thickness of 2000 Å. A portion where its source electrode and a drain electrode overlap each other is doped with phosphorus, thereby forming an N(+) type amorphous silicon semiconductor layer. Both the source electrode 40 and the drain electrode 45 are constructed of two layers. The upper layer is made of aluminum with a film thickness of 3000 Å and is formed by sputtering, and the lower layer is made of chromium with a film thickness of 600 Å and is similarly formed by sputtering. A monoplane film 50 on the TFT is a silicon nitride film with a film thickness of 1 µm formed by a plasma CVD system.

A pixel electrode 57 on the single-plane film is made of aluminum and is connected to the source electrode through a through hole 55. A high organic polyimide polymer is used as an organic alignment film 60. For a liquid crystal layer 70, HA-5073XX supplied by Chisso K.K. is used. The maximum thickness of the liquid crystal layer in the pixel electrode is set to 3.0 µm, and the retardation of the liquid crystal layer is set to 0.25 µm when no voltage is applied.

The organic alignment film 60 on the upper substrate 80 is the same as that of the lower substrate. The alignment processing directions of the organic alignment films on the upper and lower substrates are not parallel, and a pretilt angle is 5º. A common transparent pixel electrode 75 of the upper substrate is made of an indium tin oxide (ITO) film formed by sputtering, and the film thickness is 1400 Å. The upper substrate 80 is made of a non-alkaline borosilicate glass corresponding to the lower substrate, and silicon oxide layers are provided on and under the upper substrate.

A phase plate made of polysulfone and a phase plate made of polyvinyl alcohol are used as phase plate 1 PH1 and phase plate 2 PH2. The wavelength dispersion of each of the retardations of phase plates 1 and 2 and the liquid crystal layer are shown in Fig. 9.

Fig. 9 is a diagram showing the wavelength dispersion of each of the retardations of the phase plates 1 and 2 and the liquid crystal layer. The ordinate in Fig. 9 shows the retardation expressed by a ratio obtained by dividing by a value at a wavelength of 550 nm.

The retardations of the phase plates 1 and 2 are determined using a least square so that the dark display is obtained at an applied voltage of 3.0 V when the retardation of the liquid crystal layer takes a small value. The method is described below.

Equation 8 is the square of the difference between the wavelength dispersion according to Equation 2 and the total value of the retardations of the liquid crystal layer, phase plate 1 and phase plate 2 at each wavelength and shows the deviation between the wavelength dispersion of the total value of the retardations and Equation 2.

Sigma;{Δnd'(λ) - Δnd"(λ)}² S... (8)

In equation 8, Δnd"(λ) denotes the total value of the retardations of the liquid crystal layer, phase plate 1 and phase plate 2 in a similar way to equation 5. This can be rewritten as equation 9.

Δnd"(λ) = ΔndLC(λ) + RPH1·ΔndPH1(λ)/ΔndpH1

(λ = 550 nm) + RPH2 · ΔndPH2(λ)/ΔndPH2

(λ = 550 nm)... (9)

RPH1 and RPH2 in Equation 9 are the retardations of the phase plate 1 and the phase plate 2 at a wavelength of 550 nm. The signs of RPH1 and RPH2 are related to the direction of the retardation axis of the phase plate. The sign is + when the retardation axis is parallel to the alignment direction of the liquid crystal layer and - when the retardation axis perpendicularly intersects the alignment direction of the liquid crystal layer.

Equation 8 determines the sum of the five wavelengths (450 nm, 500 nm, 550 nm, 600 nm and 650 nm) in the visible wavelength range. It also determines RPH1 and RPH2, which satisfy the following two equations.

∂S/∂ RPH1 = 0... (10)

ΔndLC (λ = 550 nm) + RPH1 + RPH1 = 0.25 x 550 nm... (11)

Finally, RPH1 and RPH2, if they satisfy Equation 10 and Equation 11, minimize S in Equation 8 and yield a total value of the retardations of phase plate 1, phase plate 2, and liquid crystal layer that satisfies (coincides with) Equation 2.

Specifically, the retardation of the liquid crystal layer at a wavelength of 550 nm is 50 (nm) when the applied voltage is 3.0 V. If this value and n = 0 is replaced by ((0,25)λ, ie a quarter wavelength) and calculations are performed according to (Expression 8) to (Expression 11), RPH1 = -331 nm and RPH2 = 408 nm.

Accordingly, the retardations of the phase plate 1 and the phase plate 2 are set to 331 nm and 408 nm, respectively. The phase plate 1 and the phase plate 2 are arranged so that the retardation axis of one of them perpendicularly intersects the alignment direction of the liquid crystal layer and the other retardation axis is parallel to the alignment direction of the liquid crystal layer. A polarizing plate 95 is arranged so that the angle between its absorption axis and the alignment direction of the liquid crystal is 45°.

The total value of the retardations of the phase plate 1, the phase plate 2 and the liquid crystal layer at an applied voltage of 3.0 V is shown in Fig. 10.

Fig. 10 is a diagram showing the total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer at the time of dark display in the embodiment 1 and also the retardation curve of the quarter wavelength.

According to Fig. 10, the total value of the retardations at a wavelength of 550 nm coincides with the quarter wavelength, and in other parts of the visible wavelength range (the range of 400 to 700 nm), it is close to the quarter wavelength retardation curve. It can be said that the total value of the retardations is substantially approximated to the retardation curve. "Substantially" means that it is within the range of practical application in which even then sufficiently desirable visibility can be achieved when manufacturing tolerances are included.

The results obtained when a driving device is included in the liquid crystal display according to Embodiment 1 and the dependence of the reflectance on the applied voltage is measured at 20°C are shown in Fig. 11.

Fig. 11 is a graph showing the dependence of reflectivity on applied voltage in Embodiment 1.

The reflectivity is 0.15% when the applied voltage is 3.0 V and the contrast ratio is 100:1 or higher. A conventional contrast ratio is 28:1.

In Embodiment 1 described above, phase plates 1 and 2 having different wavelength dispersions of retardations are used, and the total value of the retardations of phase plates 1 and 2 and the liquid crystal layer are brought sufficiently close to the quarter wavelength over all visible wavelengths, whereby a display with a high contrast ratio can be achieved.

EMBODIMENT 2

According to Embodiment 2, in a liquid crystal display device consistent with that according to Embodiment 1, the retardation of the liquid crystal layer at a wavelength of 550 (nm) is set to 200 nm, and the retardations of the phase plates 1 and 2 at a wavelength of 550 (nm) are set to 322 nm and 420 nm, respectively. The total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer at an applied voltage of 3.0 V is shown in Fig. 12.

Fig. 12 is a diagram showing the total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer at the time of dark display in the embodiment 2 and the retardation curve of a quarter wavelength.

The total value corresponds to a quarter wavelength at a wavelength of 550 nm and is close to a quarter wavelength in the other sections of the visible wavelength range (400 to 700 nm). It is essentially close to a quarter wavelength.

A driving device is built into the liquid crystal display, and the results of measuring the dependence of the reflectance on the applied voltage at 20ºC are shown in Fig. 13.

Fig. 13 is a graph showing the dependence of reflectivity on applied voltage in Embodiment 2.

The reflectivity is 0.14% and the contrast ratio is 100:1 when the applied voltage is 3.0V.

As mentioned above, the total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer is determined using the phase plates 1 and 2 with different wavelength dispersions of the retardations brought sufficiently close to the quarter wavelength across all visible wavelengths, resulting in a display with a high contrast ratio.

EMBODIMENT 3

In the liquid crystal display according to Embodiment 2, the retardation of the liquid crystal layer at a wavelength of 550 (nm) is set to 200 nm, and the retardations of the phase plates 1 and 2 at a wavelength of 550 (nm) are set to 386 nm and 333 nm, respectively. The total value of the retardations of the phase plate 1, the phase plate 2, and the liquid crystal layer at an applied voltage of 1.0 V is shown in Fig. 14.

Fig. 14 is a diagram showing the total values of the retardations of the phase plates 1 and 2 and the liquid crystal layer at the time of dark display in the embodiment 3 and the retardation curve of a quarter wavelength.

The total value corresponds to a quarter wavelength at a wavelength of 550 nm and is close to and substantially approximated to a quarter wavelength over the remaining part of the visible wavelength range (the range from 400 to 700 nm).

The results of measurement of the dependence of reflectance on the applied voltage at 20°C with a driving device built into the liquid crystal display are shown in Fig. 15. Fig. 15 is a graph showing the dependence of reflectance on the applied voltage in Embodiment 3. The reflectance is 0.16%, and the contrast ratio is 100:1 when the applied voltage is 1.0 V.

As mentioned above, by using the phase plates 1 and 2 having different wavelength dispersions of the retardations, the total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer is brought sufficiently close to the fourth wavelength over all visible wavelengths, thereby obtaining a display with a high contrast ratio.

In Embodiment 3, the retardation of the phase plate 1 at a wavelength of 550 (nm) is larger than the retardation of the phase plate 2 at a wavelength of 550 (nm). Although the contrast ratio deteriorates due to a temperature change compared with Embodiments 5, 6, 8 and 9 described below, it is at a level that is not problematic compared with the conventional technique.

EMBODIMENT 4

In the liquid crystal display according to Embodiment 2, the display characteristics are measured by changing the temperature from 0°C to 50°C. The results are shown in Fig. 16. Fig. 16 is a graph showing the temperature dependence of the reflectance in a bright display, the reflectance in a dark display and the contrast ratio in Embodiment 4. Although the contrast ratio deteriorates particularly on the high temperature side, in the above-mentioned temperature range, a display with a high contrast ratio of 70:1 or more can be achieved.

EMBODIMENT 5

In the liquid crystal display according to Embodiment 1, RPH1 and RPH2 are determined in cases where (n) in Equation 2 is 4 to 0. The results are shown in Table 1. TABLE 1

In each case, the sign of RPH1 is - and the sign of RPH2 is +. The absolute value of RPH2 is greater than the absolute value of RPH1.

When the liquid crystal display of the present invention is designed such that a phase plate whose gradient of wavelength dispersion of retardation is shallower than that of the liquid crystal layer is used as the phase plate 1, a phase plate whose gradient of wavelength dispersion of retardation is shallower than that of the liquid crystal layer is used as the phase plate 2, and (n) in the equation 2 is 0 or larger, the phase plate 1 is arranged so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal layer, and the phase plate 2 is set so that the retardation axis is parallel to the alignment direction of the liquid crystal layer. When the retardation of the phase plate 2 is set to be larger than that of the phase plate 1, a display with a high contrast ratio can be obtained.

EMBODIMENT 6

In the liquid crystal display according to Embodiment 1, RPH1 and RPH2 are determined in cases where (n) in Equation 2 is -1 to -4. The results are shown in Table 1.

In any case, the sign of RPH1 is + and the sign of RPH2 is -. The absolute value of RPHa is greater than the absolute value of RPH1.

When the liquid crystal display according to the invention is constructed such that a phase plate whose gradient of the wavelength dispersion of the retardation is steeper than that of the liquid crystal layer is used as the phase plate 1, a Phase plate whose gradient of wavelength dispersion of retardation is flatter than that of the liquid crystal layer is used as the phase plate 2 and (n) in the equation 2 is set to -1 or less, the phase plate 1 is arranged so that the retardation axis is parallel to the alignment direction of the liquid crystal layer and the phase plate 2 is set so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal layer. When the retardation of the phase plate 2 is set to be larger than that of the phase plate 1, a display with a high contrast ratio can be achieved.

EMBODIMENT 7

This relates to a case where the polarizing plate is replaced with a polarizing beam splitter in the liquid crystal display according to Embodiment 1. That is, it relates to the case of setting substantially approximating the retardation curve expressed by the relationship (0.5 n)λ. When Δnd'(λ) in Equation 8 is set to the wavelength dispersion of the retardation defined by Equation 7 and n = 0 (wavelength 0), RPH1 and RPH2 are obtained. As shown in Table 2, RPH1 = -21 nm and RPH2 = -29 nm. TABLE 2

Therefore, the retardations of the phase plates 1 and 2 at wavelengths of 550 (nm) are set to 21 nm and 29 nm, respectively, a phase plate whose gradient of wavelength distribution of retardation is steeper than that of the liquid crystal layer is used as the phase plate 1, a phase plate whose gradient of wavelength dispersion of retardation is shallower than that of the liquid crystal layer is used as the phase plate 2, (n) is set to 0 in the equation 7, and the retardation axes of both are arranged to perpendicularly intersect the alignment direction of the liquid crystal layer.

When a driving device is built into the liquid crystal display and the dependence of the reflectance on the applied voltage is measured at 20ºC, the reflectance is 0.12% when the applied voltage is 3.0 V and the contrast ratio is 100:1 or higher.

As mentioned above, phase plates 1 and 2 each having a different wavelength dispersion of retardation are used, and the total value of the retardations of the phase plates 1 and 2 and the liquid crystal layer is determined via all visible wavelengths are brought sufficiently close to zero wavelength, resulting in a display with a high contrast ratio.

EMBODIMENT 8

In the liquid crystal display according to Embodiment 7, RPH1 and RPH2 are determined in cases where (n) in Equation 7 is 4 to 1. The results are shown in Table 2.

In each case, the sign of RPH1 is - and the sign of RPH2 is +. The absolute value of RPH2 is greater than the absolute value of RPH1.

When the liquid crystal display according to the present invention is designed such that a phase plate whose gradient of wavelength dispersion of retardation is steeper than that of the liquid crystal layer is used as the phase plate 1, a phase plate whose gradient of wavelength dispersion of retardation is shallower than that of the liquid crystal layer is used as the phase plate 2, and (n) in the equation 2 is set to 1 or more, the phase plate 1 is arranged so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal layer, and the phase plate 2 is set so that the retardation axis is parallel to the alignment direction of the liquid crystal layer. When the retardation of the phase plate 2 is set to be larger than that of the phase plate 1, a display with a high contrast ratio can be achieved.

EMBODIMENT 9

In the liquid crystal display according to Embodiment 7, RPH1 and RPH2 are determined in cases where (n) in Equation 7 is -1 to -4. The results are shown in Table 2.

In each case, the sign of RPH1 is + and the sign of RPH2 is -. The absolute value of RPH2 is greater than the absolute value of RPH1.

Therefore, when the liquid crystal display according to the present invention is designed such that a phase plate whose gradient of wavelength dispersion of retardation is steeper than that of the liquid crystal layer is used as the phase plate 1, a phase plate whose gradient of wavelength dispersion of retardation is shallower than that of the liquid crystal layer is used as the phase plate 2, and (n) in the equation 2 is set to -1 or less, the phase plate 1 is arranged so that the retardation axis is parallel to the alignment direction of the liquid crystal layer, and the phase plate 2 is set so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal layer. When the retardation of the phase plate 2 is set to be larger than that of the phase plate 1, a display with a high contrast ratio can be achieved.

COMPARISON EXAMPLE

In the liquid crystal display according to Embodiment 2, the phase plate 2 is omitted, and the retardation of the phase plate 1 at a wavelength of 550 (n) is set to 174 nm The total value of the retardations of the phase plate 1 and the liquid crystal layer at an applied voltage of 3.0 V is shown in Fig. 17. The total value corresponds to the quarter wavelength at the wavelength of 550 nm, but is far from the quarter wavelength in the other portions of the visible wavelength region.

The driving device is built into the liquid crystal display, and the dependence of the reflectance on the applied voltage at 20ºC is measured. The results are shown in Fig. 18. The reflectance is 1.05% when the applied voltage is 3.0 V, and the contrast ratio is 28:1.

As mentioned above, when using only one phase plate, the total value of the retardations cannot be brought sufficiently close to the quarter wavelength over all visible wavelengths and a high contrast ratio display cannot be achieved.

According to the embodiments of the invention, the total value of the retardations of the liquid crystal layer and the phase plates 1 and 2 is brought close to the quarter wavelength over the entire visible wavelength range, and a display with a high contrast ratio can be achieved. Accordingly, a reflection type liquid crystal display with a high contrast ratio and a high aperture ratio can be realized.

The liquid crystal layer according to the embodiment has a "layer structure with homogeneous alignment". However, even when using a "layer structure with twist", by determining the retardation axis of the phase plate and the transmission axis of the polarizing plate using the alignment direction of the liquid crystal layer on the side near the polarizing plate as a reference, similar effects can be achieved.

The phase plate and the polarizing plate are not limited to the organic film made of a high polymer. Similar effects can also be achieved by using a phase plate and a polarizing plate made of an inorganic material. Furthermore, similar effects can also be achieved by using a polysilicon TFT or a MOS as an active element.

Next, a liquid crystal display element used for a projection type liquid crystal display and, in particular, a method for controlling a wavelength dispersion of a retardation of a reflection plate-built-in type liquid crystal display element using a birefringence mode will now be described.

First, the reason why a display is colored at a low contrast in a conventional projection type liquid crystal display is considered. In the projection type liquid crystal display, a transmission component (linear polarization) is selected from a light source using a polarization beam splitter. The reflectance R(λ) of the light having the wavelength λ in this case is expressed by the following equation.

R(λ) = 0.25(1 - cos(δ(λ)) ... (12)

When the total value of the retardations of the liquid crystal layer and the phase plate is set to Δndto(λ), (δ(λ) is expressed by the following equation.

(δ(λ) = 2π·2Δndto(λ)/λ... (13)

In equation 13, the "2" before 2Δndto(λ) indicates that the light is reflected by the reflection plate and passes through the phase plate and the liquid crystal layer twice. To obtain high contrast when R(λ) = 0, the following equation must be satisfied over the entire visible wavelength range.

2Δndto(λ) = 0.5nλ ... (14)

where n is an integer. When Equation 14 is plotted for cases where n = -2, -1, 0, 1, 2, Equation 14 gives a linear line as shown in Fig. 20 when n has any of the above values. When n ≥ 0, (n) increases monotonically with wavelength. When n < 0, n decreases monotonically with wavelength. On the other hand, the retardations of the phase plate and the liquid crystal layer are significantly different from those in Fig. 20 and the downward convex curves shown in Fig. 21 and decrease monotonically with wavelength.

The case of n = 0 is taken as an example, and an attempt is made to make Δndto(λ) close to Equation 14 by optimizing the retardations of the phase plate and the liquid crystal layer and the layering method over the entire visible wavelength range. Equation 15 shows Δndto(λ) of the liquid crystal layer and one phase plate, and Equation 16 shows Δndto(λ) of the liquid crystal layer and two phase plates.

Δndto(λ) = ΔndLC(λ) ± ΔndPH(λ)... (15)

Δndto(λ) = ΔndLC(λ) ± ΔndPH1(λ) ± ΔndPH2(λ) ... (16)

ΔndLC(λ) denotes the retardation of the liquid crystal layer. ΔndPH1(λ), ΔndPH2(λ) and ΔndPH(λ) are the retardations of the Phase plates. The double signs in equations 15 and 16 are not sequential. When the retardation axis of the phase plate is parallel to the alignment direction of the liquid crystal, the sign is +. When the retardation axis intersects the alignment direction perpendicularly, the sign is -. An example of Δndto(λ) of the liquid crystal layer and a phase plate is shown in Fig. 22. ΔndLC(λ) and ΔndPH(λ) are determined so that Δndto(λ) coincides with equation 14 at a wavelength of 550 nm. Although the retardation increases monotonically with wavelength, it is close to equation 14 only around the wavelength of 550 nm. An example of Δndto(λ) of the liquid crystal layer and a phase plate is shown in Fig. 23. Although an extremely close line is obtained that matches at a wavelength of 550 nm, it approximates Equation 14 only around the wavelength of 550 nm.

The reflection spectrum of the dark display obtained by the wavelength dispersion of the retardations according to the equation 16 is shown in Fig. 24. Although the reflectance is sufficiently reduced around the wavelength of 550 nm where the total value of the retardations is close to the equation 14, the reflectance is not sufficiently reduced except for the above. Therefore, high contrast cannot be obtained. Since the reflection spectrum of the black display is not flat, black on the display is colored. Since the displayed colors change continuously on color coordinates, a gray display near the black display is also colored. The wavelength dispersion of both the phase plate and the single polarizing plate always gives a downward convex curve. Accordingly, the wavelength dispersion of the retardation even at a Increasing the number of phase plates to three or more always produces an upward convex curve, and the result is the same as that of two phase plates. As mentioned above, the low contrast and color display in the conventional projection type liquid crystal display occur because the wavelength distribution of retardations required for a dark display cannot be realized in principle over the entire visible range.

Accordingly, the colors R, G, B are displayed by different liquid crystal display elements and optically added, thereby obtaining a color display. The wavelength ranges of the light passing through the liquid crystal display elements are different. The width of the wavelength range is narrowed to about 1/3 of the entire visible wavelength range. Therefore, the retardations of the phase plates of the liquid crystal display elements are set to different values so that Equation 14 is satisfied at the center wavelength of each color. The total value of the retardations at this time is, as shown in Fig. 19, an overlap of three retardations of different wavelengths (the wavelengths of the colors R, G and B) which are close to 0.5λ. Since the reflectance of the black display is reduced for all visible wavelengths, a high contrast can be obtained. The reflection spectrum obtained at this time is approximately flat for all visible wavelengths because the retardations of the three wavelengths close to 0.5λ become minimum, as shown in Fig. 25. The black display accordingly becomes colorless, and the gray display near the black display also becomes approximately colorless.

The projection type liquid crystal display according to the embodiments of the invention will be described in more detail with reference to the accompanying drawings.

EXAMPLE 10

The structure of a light source, an optical system and liquid crystal display elements of the liquid crystal display useful for understanding the invention is shown in Fig. 26. The light source LS is a metal halide lamp for directing light from the light source to a bicolor mirror DC 1 using a group CL of condenser lenses. The light from the light source is distributed by the bicolor mirror DC into the three colors R, G and B, which are directed to different liquid crystal display elements LCR, LCG and LCB via a polarization beam splitter. The wavelength ranges of the light incident on the liquid crystal display elements LCR, LCG and LCB are 600 to 700 nm, 500 to 600 nm and 400 to 500 nm, respectively. The reflected light from the liquid crystal display elements LCR, LCG and LCB is synthesized by the two-color mirror DC 2, thereby performing the color display. The color display is projected onto a screen SL using the group PL of projection lenses.

The structure of the liquid crystal display elements LCR, LCG and LCB is the same except for the phase plates 1 and 2 described later. The cross section of a liquid crystal display element is shown in Fig. 27. A pixel is constructed of a pixel electrode AL and also serves as a reflection plate. The pixel electrode AL is connected to a wiring layer LI and a MOS through through holes. An alignment film OL is formed on the pixel electrode AL is formed. An upper substrate has a transparent electrode LI and the alignment film OL. The phase plate 1 and the phase plate 2 are laminated on the upper substrate.

A nematic fluorine liquid crystal material is used for the liquid crystal layer. The birefringence at 20ºC is 0.083. The thickness of the liquid crystal layer is 2.4 µm, the retardation of the liquid crystal layer is set to 200 nm when no voltage is applied. The alignment of the liquid crystal layer is set to a homogeneous alignment, and a pretilt angle is set to 4º.

A phase plate made of polyvinyl alcohol and a phase plate made of polysulfone are used as phase plate 1 and phase plate 2, respectively. Fig. 21 shows the wavelength dispersion of the retardations of both the polyvinyl alcohol plate and the polysulfone plate and the nematic fluorine liquid crystal material. The ordinate in Fig. 21 shows the retardations standardized by the value of a wavelength of 550 nm. They all give downward-convex curves that decrease monotonically with an increase in the wavelength.

The retardations of phase plates 1 and 2 laminated on LCR, LCG and LCB, respectively, are determined as shown below. First, LCR is set to a wavelength of 650 nm as a reference. LCG is set to a wavelength of 550 nm as a reference. LCB is set to a wavelength of 450 nm as a reference. Each of these wavelengths is the center wavelength of the wavelength range of the light incident on each liquid crystal display element. The voltage at the time of dark display is set to 3 V. The Retardation of the liquid crystal layer is 38 nm for LCR, 40 nm for LCG and 42 nm for LCB when 3 V is applied.

With respect to LCR, the delays of phase plates 1 and 2 are determined such that equation 17 is minimized.

Σ(0.5λ - Δndto(λ))² ... (17)

Δndto(λ) = ΔndLC(λ) ± RPH1 ΔndpPH1'(λ) + RPH2 ΔndPH2'(λ) ... (18)

0.5λ in equation 17 is one of the conditions for obtaining the dark display and corresponds to a case where n = 1 in equation 14. RPH1 and RPH2 in equation 18 are the retardations of phase plates 1 and 2 to be determined. ΔndPH1'(λ) and ΔndPH2'(λ) are the retardations of phase plates 1 and 2 standardized by a wavelength of 650 nm.

ΔndLC(λ) is the retardation of the liquid crystal layer at the wavelength and is equal to 38 nm at a wavelength of 650 nm, as mentioned above. Equation 17 calculates the sum of the values at wavelengths of 600, 610, 620, 630, 640, 650, 660, 670, 680, 690 and 700 nm. The result is RPH1 = 1233 nm and RPH2 = -946 nm.

Since the sign of RPH1 is positive, the phase plate 1 is layered so that the retardation axis is parallel to the alignment direction of the liquid crystal. Since the sign of RPH2 is negative, the phase plate 2 is layered so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal.

The delays of phase plates 1 and 2 to be stacked on LCG and LCB, respectively, are determined similarly. For LCG, ΔndPH1'(λ) and ΔndPH2'(λ) are each set to Wavelength of 550 nm standardized delays are set. Equation 17 calculates the sum of the values at wavelengths of 500, 510, 520, 530, 540, 550, 560, 570, 580, 590 and 600 nm. The results are RPH1 = 862 nm and RPH2 = -627 nm. For LCB, ΔndPH1'(λ) and ΔndPH2'(λ) are set to delays standardized by a wavelength of 450 nm. Expression 6 calculates the sum of the values at wavelengths of 400, 410, 420, 430, 440, 450, 460, 470, 480, 490 and 500 nm. The results are RPH1 = 554 nm and RPH2 = -371 nm. Since the signs of RPH1' and RPH2 for LCG and LCB are the same as for LCR, the procedure for stacking phase plates 1 and 2 on LCG and LCB is the same as for LCR.

Fig. 28 shows the total value of the retardations of the liquid crystal layer, the phase plate 1 and the phase plate 2 at the time of dark display (when 3 V is applied) for LCR, LCG and LCB. Light having wavelength ranges of 600 to 700 nm, 500 to 600 nm and 400 to 500 nm are mainly incident on LCR, LCG and LCB, respectively. Therefore, in Fig. 28, the values of LCR in the wavelength range of 600 to 700 nm, the values of LCG in the wavelength range of 500 to 600 nm and the values of LCB in the wavelength range of 400 to 500 nm are shown. Since the total value of the retardations through the three wavelengths (650, 550 and 450 nm) is close to 0.5? , it is close to 0.5λ in a wavelength range of 400 to 700 nm. The LCR, LCG and LCB elements constructed as described above were incorporated into the projection type display and the display characteristics were evaluated. The measurement was carried out in a dark room to exclude the influence of external light. The same voltage was applied to the LCR, LCG and LCB, the display was projected onto a screen 2 m away. projected, and brightness and hue were measured. The dependence of the brightness of the screen on the applied voltage is shown in Fig. 29. The minimum value of 1.9 cd/m² was found at an applied voltage of 3.0 V, and the maximum value of 250 cd/m² was found at an applied voltage of 1.2 V. The contrast ratio was 130:1. Fig. 30 shows a map of the dependence of hue on the applied voltage on the UCS color coordinate system. The hue of the display is distributed between the maximum and minimum values for the screen brightness near a light source C, and the colors are approximately achromatic. Even in the case of maximum coloration, the distance from the light source C is 0.03.

As mentioned above, by setting the different retardations of the phase plates of the three liquid crystal display elements to different values according to the wavelengths of the incident light, a high-contrast achromatic display was realized.

EXAMPLE 11

In the projection type liquid crystal display shown in Example 10, the delays of the phase plates are readjusted by adjusting the voltage at the time of the dark display to 1 V.

The retardations of the liquid crystal layer are 189 nm for LCR, 195 nm for LCG and 205 nm for LCB when 1 V is applied. ΔndLC(λ) is set to 189 nm at a wavelength of 650 nm in Equation 17, and ΔndPH1'(λ) and ΔndPH2'(λ) are set to retardations standardized at a wavelength of 650 nm. to determine RPH1 and RPH2 for LCR. Similarly, the retardations of phase plates 1 and 2 stacked on LCG and LCB, respectively, are determined. The results are RPH1 = 1137 nm and RPH2 = -996 nm for LCR, RPH1 = 776 nm and RPH2 = -691 nm for LCG, and RPH1 = 465 nm and RPH2 = -440 nm for LCB.

The LCR, LCG and LCB elements formed as described above were incorporated into the projection type display, and the display characteristics were evaluated. The minimum value of 2.1 cd/m² was obtained at an applied voltage of 1.0 V, and the maximum value of 255 cd/m² was obtained at an applied voltage of 2.3 V. The contrast ratio was 121:1. The hue of the display was distributed between the maximum and minimum values of the screen brightness near the light source C, and the colors were approximately achromatic. Even in the case of maximum chromaticity, the distance from the light source C was 0.03.

As mentioned above, by setting the retardations of the phase plates of the three liquid crystal display elements to different values according to the wavelengths of the incident light, an achromatic display with high contrast can be realized.

EXAMPLE 12

When displaying the projection type according to Example 10, the delays of phase plates 1 and 2 are determined by changing equation 17 to the following expression.

Σ(-0.5λ - Δndto(λ))² ... (19)

- 0.5&lambda; in expression 8 corresponds to a case where n = -1 in equation 19. For LCR, RPH1 is -1283 nm and RPH2 = 920 nm, for LCG, RPH1 is -908 nm and RPH2 = 593 nm, and for LCB, RPH1 is -601 nm and RPH2 is 334 nm.

For both LCR and LCG and LCB, the sign of RPH1 is negative and the sign of RPH2 is positive. Therefore, for both LCR and LCG and LCB, the phase plate 1 is stacked so that the retardation axis perpendicularly intersects the alignment direction of the liquid crystal, and the phase plate 2 is stacked so that the retardation axis is parallel to the alignment direction of the liquid crystal.

The LCR, LCG and LCB elements constructed as above were incorporated into the projection type display and the display characteristics were evaluated. The minimum value of 2.0 cd/m² was obtained at an applied voltage of 3.0 V and the maximum value of 249 cd/m² was derived at an applied voltage of 1.3 V. The contrast ratio was 125:1. The hue of the display is distributed between the maximum and minimum values of the screen brightness near the light source C and the colors are approximately achromatic. Even in the case of the greatest colorfulness, the distance from the light source C was 0.03.

As mentioned above, by setting the retardations of the phase plates of the three liquid crystal display elements to different values corresponding to the wavelengths of the incident light, achromatic display with high contrast can be realized.

COMPARISON EXAMPLE

In the projection type display according to Example 10, the delays of the phase plates 1 and 2 of each of the elements LCR and LCB are set equal to those of LCG. This means that the delays of the phase plates of all the liquid crystal display elements used for the projection type display are set equal.

Fig. 33 shows the total values at the time of a dark display. Although the total values are close to 0.5 λ near a wavelength of 550 nm, they are far from 0.5 λ at both ends of the visible wavelength range.

Fig. 31 shows the dependence of the screen brightness on the applied voltage. The minimum value of 5.2 cd/m² was determined at an applied voltage of 3.0 V, and the maximum value of 240 cd/m² was derived at an applied voltage of 1.2 V. The contrast ratio was 46:1. Fig. 32 shows the dependence of the hue on the applied voltage mapped on the UCS color coordinate system. Although the hue of the display once approaches the light source C between the maximum and minimum values of the screen brightness, it is far away at the minimum value. The distance from the light source C is 0.08 at the minimum value of the reflectance, and gray appears purple at the time of dark display on the low luminance side.

As mentioned above, if the retardations of the phase plates of the three liquid crystal display elements are set to the same value without taking into account the wavelengths of the incident light, the total value of the retardations except at one point in the visible Wavelength range is not set for the conditions of equation 14. Therefore, even when a dark display is made, light is transmitted on both sides of the visible wavelength range, and the dark display is colored purple. This deteriorates the contrast ratio, and gray is colored on the low luminance side at the time of a dark display.

According to the above-described embodiments of the invention, a projection type liquid crystal display capable of displaying in achromatic colors and with high contrast is realized. Although the layer structure of the liquid crystal layer had a homogeneous alignment, the invention is not limited to such an alignment. Even when a layer structure having a twist is used, similar effects can be achieved by determining the retardation axis of the phase plate and the transmission axis of the polarizing plate using the alignment direction of the liquid crystal adjacent to the polarizing plate as a reference. The phase plate and the polarizing plate are not limited to the use of high organic polymer films. Similar effects can also be achieved by using phase and polarizing plates made of an inorganic material.

Claims (7)

1. Liquid crystal display comprising a transparent substrate (80), a substrate (10), a liquid crystal layer (70) located between the transparent substrate and the substrate, and electrodes (40, 45, 57, 75) mounted on the transparent substrate and the substrate, a voltage being applied between them for controlling the liquid crystal layer during display, the liquid crystal display being of the reflection type in which incident light penetrates the transparent substrate and is reflected by a reflector mounted on the substrate, the light transmittance being controlled by the liquid crystal layer, and with two phase plates (PH1, PH2), wherein the two phase plates have different values of the gradient of the wavelength dispersion of the retardation such that the total value of the retardation of the two phase plates in combination with the liquid crystal layer substantially approximates a retardation curve expressed by the relationship (0.25 + 0.5n)λ, where n is an integer and λ is the wavelength (nm) of the transmitted light, characterized in that the two phase plates (PH1, PH2) are arranged on the transparent substrate on the side opposite the liquid crystal layer, and a polarization plate (95) is arranged on the two phase plates, which has an absorption axis which is aligned with the alignment direction the liquid crystal layer on the side of the polarizing plate is non-parallel. 2. A liquid crystal display according to claim 1, wherein the relationship of the total value of the wavelength dispersion of the retardation of the two-phase plates in combination with the liquid crystal layer is applicable in the wavelength range of light from 400 nm to 700 nm. 3. Liquid crystal display with a transparent substrate (80), a substrate (10), a liquid crystal layer (70) between the transparent substrate and the substrate, and electrodes (40, 45, 57, 75) on the transparent substrate and the substrate, wherein a voltage is applied between them for the display to control the liquid crystal layer, and further with two phase plates (PH1, PH2), characterized in that the two phase plates (PH1, PH2) are arranged on the transparent substrate on the side opposite to the liquid crystal layer, a polarizing device (95) having an absorption axis that is non-parallel to an alignment direction of the liquid crystal layer on the side near the polarizing plate is arranged on the two phase plates; wherein the polarizing means is a polarizing beam splitter which rotates the directions of oscillation of a polarized component of transmitted light by 90° when light passing through the polarizing plate changes direction; and the two-phase plates have different values of the gradient of the wavelength dispersion of the retardation such that the total value of the retardation of the two phase plates in combination with the liquid crystal layer is substantially is approximately equal to a retardation curve expressed by the relationship (0.5n)λ in the light wavelength range from 400 nm to 700 nm, where n is an integer and λ is the wavelength of the transmitted light. 4. A liquid crystal display according to claim 3, wherein the integer n is greater than 0; the gradient of the wavelength dispersion of the retardation of a phase plate (1) arranged in the vicinity of the polarization plate is steeper than that of the wavelength dispersion of the retardation of the liquid crystal layer; the gradient of the wavelength dispersion of the retardation of the phase plate (2) in the liquid crystal layer is flatter than that of the wavelength dispersion of the retardation of the liquid crystal layer; the phase plate (1) has a delay phase axis which is perpendicular to the alignment direction of the liquid crystal layer on the side on which the two-phase plates are arranged; and the phase plate (2) has a delay phase axis which is parallel to the alignment direction of the liquid crystal layer on the side on which the two phase plates are arranged, wherein the retardation of the phase plate (2) at a wavelength of 550 nm is greater than a retardation of the phase plate (1) at a wavelength of 550 nm. 5. A liquid crystal display according to claim 3, wherein the integer n is greater than 1; the gradient of the wavelength dispersion of the retardation of the phase plate (1) near the polarization plate is steeper than the gradient of the wavelength dispersion of the retardation of the liquid crystal layer; the gradient of the wavelength dispersion of the retardation of the phase plate (2) in the vicinity of the liquid crystal layer is flatter than the gradient of the wavelength dispersion of the retardation of the liquid crystal layer; the phase plate (1) has a retardation phase axis which is perpendicular to the alignment direction of the liquid crystal layer on the side of the two-phase plates; the phase plate (2) has a retardation phase axis which is parallel to the alignment direction of the liquid crystal layer on the side of the two-phase plates, and wherein the retardation of the phase plate (2) at a wavelength of 550 nm is greater than the retardation of the phase plate (1) at a wavelength of 550 nm. 6. A liquid crystal display according to claim 3, wherein the integer is 0 ; the gradient of the wavelength dispersion of the retardation of the phase plate (1) at the polarization plate is steeper than the gradient of the wavelength dispersion of the retardation of the liquid crystal layer; and the gradient of the wavelength dispersion of the retardation of the phase plate (2) in the liquid crystal layer is flatter than the gradient of the wavelength dispersion of the retardation of the liquid crystal layer, and wherein the delay phase axes of the phase plate (1) and the phase plate (2) are arranged at right angles to the alignment direction of the liquid crystal layer on the side of the two-phase plates. 7. A liquid crystal display according to claim 2 or 3, wherein the integer n is less than 0; the gradient of the wavelength dispersion of the retardation of the phase plate (1) at the polarization plate is steeper than the gradient of the wavelength dispersion of the retardation of the liquid crystal layer; the gradient of the wavelength dispersion of the retardation of the phase plate (2) in the liquid crystal layer is flatter than the gradient of the wavelength dispersion of the retardation of the liquid crystal layer; the phase plate (1) has a retardation phase axis which is parallel to the alignment direction of the liquid crystal layer on the side of the two-phase plates; the phase plate (2) has a retardation phase axis which is perpendicular to the alignment direction of the liquid crystal layer on the side of the two-phase plates, and wherein the retardation of the phase plate (2) at a wavelength of 550 nm is greater than the retardation of the phase plate (1) at a wavelength of 550 nm.